Multi-particle correlations measured in all collision systems, contain an imprint of the initial state of the colliding partners, characterised via their quark and gluon degrees of freedom, allowing the extraction of fundamental properties of the quark–gluon plasma [1]}, [2]}, [3]}.
Hydrodynamic-like (final-state) collective flow is increasingly part of the modelling of small collision systems [4]}, [5]} in an interplay with initial-state phenomena (see reviews in Refs. [6]}, [7]}).
Such collision systems are usually modelled in the framework of the colour glass condensate (CGC) [8]}, where multi-particle production proceeds via the decay of colour flux tubes that stretch between two colliding hadrons in the longitudinal direction and are coherent in the transverse direction over a range that is inversely proportional to a saturation scale \(Q_{\textrm {s}}\) , which appears due to the non-linearity of parton evolution at high energies.
